Treatment resistant motor features, such as postural instability and freezing of gait are common in people with Parkinson's disease and major contributors to morbidity and mortality. This project will use sleep studies, quantitative motor assessments and magnetic resonance imaging to examine the relationship between abnormally increased muscle activity during rapid eye movement sleep and the development and progression of treatment-resistant postural instability and gait disturbances. Specifically, the investigators will test the hypothesis that anticipatory postural adjustments (weight and pressure shifts) during gait initiation are significantly reduced in people with Parkinson's disease who have abnormally elevated muscle activity during rapid eye movement (REM) sleep compared to individuals will Parkinson's disease whose REM sleep muscle activity is normal. In addition, the investigators will test the hypothesis that the level of RSWA at baseline is predictive of measures of motor decline (postural stability and gait) and alterations in the structure and function of locomotor brainstem networks. Since sleep disorders can emerge years before a diagnosis of Parkinson's disease, establishment of a link between sleep and treatment-resistant posture and gait disorders will help identify individuals at risk of developing these disabling motor features of disease.